A common type of filter system for removing a small amount of contaminants (usually less than one-percent by volume) from otherwise pure liquid, employs a disposable filter having a long cardboard tube at the filter center. A long sheet of filter material is wound in a tight spiral around the cardboard tube, to form an elongated cylindrical filter with a central passage. Such filters are commonly used to filter oil in vehicles, where a filter lies in a small pressure chamber containing a single filter, with oil pumped into the chamber and with oil passing through the filter then passing out through an outlet pipe to be reused. High efficiency and low cost filters are available in a range of sizes for vehicles such as trucks. The largest commonly available filter of this type is rated for 5 gpm (gallons per minute, for fluid of very low viscosity). Very good filters of this type block contaminants of a size less than 0.5 microns. Contaminants build up in the filter, until it is disposed of.
There are applications which require a much higher flow rate than 5 gpm, such as the oil systems of larger vessels, small community water purification systems, etc. Large filters having a capacity a plurality of times greater than 5 gpm are available for special applications. However, such large filters are much more expensive per unit capacity than the commonly available filters for vehicle oil systems, and these larger filters are not as efficient as the better vehicle oil filters. It would be possible to use a large number of vehicle oil filter systems connected in parallel, so the low cost and highly efficient 5 gpm filters could be used. However, the cost for a large number of pressure vessels is high, maintenance costs would be relatively high because of the need to open each of many pressure vessels to replace the filters, and the possibility of fluid leakage would increase because of the numerous vessels and connections to each of them. A high capacity liquid filter system which could use readily available low cost and efficient filters, and which had a low possibility of fluid leakage while being maintainable at moderate cost, would be of value.